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        <h3 id="前言"><a href="#前言" class="headerlink" title="前言"></a>前言</h3><p>在上篇<a href="/2019/10/10/分析-Vue-的观察者模式（上）/" title="分析 Vue 的观察者模式（上）">分析 Vue 的观察者模式（上）</a>中，简单实现了“手动”版观察者模式，而在本篇中我们继续沿着该思路往下探索，看如何实现“自动”版的观察者。</p>
<a id="more"></a>
<h3 id="完善的观察者"><a href="#完善的观察者" class="headerlink" title="完善的观察者"></a>完善的观察者</h3><p>紧接着上文的思路，因为我们只是实现了 Dep 依赖（管家），却还没有对平台的观察处理方式优化，因此我们需要将此步骤给完善起来。</p>
<p>首先，要实现发布者更新内容后自动触发订阅事件，那么我们需要的是对发布者内容改变的监听，ES5 中恰好有这么一种方法能满足我们的需求，那就是 <code>Object.defineProperty(...)</code>。其语法很简单，我们此处仅仅需要拦截 get 和 set 方法，大致写法为：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">const</span> data = &#123;</span><br><span class="line">  name: <span class="string">'test'</span></span><br><span class="line">&#125;;</span><br><span class="line"></span><br><span class="line"><span class="built_in">Object</span>.defineProperty(data, name, &#123;</span><br><span class="line">  <span class="keyword">get</span>() &#123;</span><br><span class="line">    <span class="keyword">return</span> data.name;</span><br><span class="line">  &#125;,</span><br><span class="line">  <span class="keyword">set</span>(newVal) &#123;</span><br><span class="line">    <span class="comment">// 调用 setter 方法，不可直接赋值，否则会死循环</span></span><br><span class="line">    <span class="comment">// 错误写法 obj[key] = newVal</span></span><br><span class="line">    <span class="comment">// 可查看 Vue 中的 setter 获取</span></span><br><span class="line">  &#125;</span><br><span class="line">&#125;)</span><br></pre></td></tr></table></figure>
<p>既然有了能够观测发布者的方法，那么对于对象变更后的监听就能自动化处理了，那么先前手动触发 <code>dep.depend()</code> 和 <code>dep.notify()</code> 函数的操作就能整合入 <code>Object.defineProperty</code> 内了。</p>
<p>延续先前的逻辑，<code>dep.notify()</code> 方法执行都得在对象值变动之后，因此我们可以将此方法放入 set 函数内；而 <code>dep.depend()</code> 方法主要是将订阅者事件存储入自身的事件列表中，因此该方法执行一次就行，之后每次更新都触发 <code>notify</code> 来依次执行注册的函数即可。因此，在考虑只是传入简单对象的情况时，这阶段的代码思路大致是：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// （私人管家）依赖收集</span></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Dep</span> </span>&#123;</span><br><span class="line">  <span class="keyword">constructor</span>() &#123;</span><br><span class="line">    <span class="keyword">this</span>.subscriberList = [];</span><br><span class="line">  &#125;</span><br><span class="line"></span><br><span class="line">  <span class="comment">// 添加订阅方法</span></span><br><span class="line">  depend() &#123;</span><br><span class="line">    <span class="comment">// 此处添加对应订阅方法</span></span><br><span class="line">  &#125;</span><br><span class="line"></span><br><span class="line">  <span class="comment">// 发布者更新消息</span></span><br><span class="line">  notify() &#123;</span><br><span class="line">    <span class="keyword">this</span>.subscriberList.forEach(<span class="function"><span class="params">sub</span> =&gt;</span> &#123;</span><br><span class="line">      <span class="comment">// 依次处理订阅的方法</span></span><br><span class="line">    &#125;);</span><br><span class="line">  &#125;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="comment">// （发布者和管家的联系方式）监听器</span></span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">observer</span>(<span class="params">data = &#123;&#125;</span>) </span>&#123;</span><br><span class="line">  <span class="built_in">Object</span>.keys(data).forEach(<span class="function"><span class="params">key</span> =&gt;</span> &#123;</span><br><span class="line">    <span class="keyword">let</span> val = data[key];</span><br><span class="line">    <span class="keyword">const</span> dep = <span class="keyword">new</span> Dep();</span><br><span class="line"></span><br><span class="line">    <span class="built_in">Object</span>.defineProperty(data, key, &#123;</span><br><span class="line">      <span class="keyword">get</span>() &#123;</span><br><span class="line">        <span class="comment">// get 时添加依赖，当有目标的时候才添加</span></span><br><span class="line">        <span class="keyword">if</span> (target) &#123;</span><br><span class="line">          dep.depend();</span><br><span class="line">        &#125;</span><br><span class="line">        <span class="keyword">return</span> val;</span><br><span class="line">      &#125;,</span><br><span class="line">      <span class="keyword">set</span>(newVal) &#123;</span><br><span class="line">        <span class="keyword">if</span> (newVal === val) &#123;</span><br><span class="line">          <span class="keyword">return</span>;</span><br><span class="line">        &#125;</span><br><span class="line">        <span class="comment">// set 时触发更新</span></span><br><span class="line">        <span class="comment">// 此处用 setter 做变动，此处偷懒一下，不要直接赋值，会反复触发 set</span></span><br><span class="line">        val = newVal;</span><br><span class="line">        dep.notify();</span><br><span class="line">      &#125;</span><br><span class="line">    &#125;)</span><br><span class="line">  &#125;);</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>那么现在问题又有了。目前代码实现至此我们仅仅只是将发布者与其 Dep 依赖进行了关联，那么观察者又该如何将其获订阅者的事件与 Dep 存储的事件列表进行关联呢？从订阅者角度分析一下：</p>
<ol>
<li>订阅者决定好要订阅的内容（订阅者的函数）；</li>
<li>订阅者在对应的观察者平台上注册事件，绑定发布者信息；</li>
<li>观察者先联系到 Dep 管家，将订阅事件存放至管家处；</li>
<li>待管家拿到了发布者新情报，则响应所有存储好的订阅者的订阅事件了。</li>
</ol>
<p>思路逐渐清晰，那么这么个流程我们可以简化为如下代码：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br></pre></td><td class="code"><pre><span class="line">target = <span class="literal">null</span>;</span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Dep</span> </span>&#123;</span><br><span class="line">  <span class="keyword">constructor</span>() &#123;</span><br><span class="line">    <span class="keyword">this</span>.sub = [];</span><br><span class="line">  &#125;</span><br><span class="line">  depend() &#123;</span><br><span class="line">    <span class="keyword">if</span> (target &amp;&amp; !<span class="keyword">this</span>.sub.includes(target)) &#123;</span><br><span class="line">      <span class="keyword">this</span>.sub.push(target);</span><br><span class="line">    &#125;</span><br><span class="line">  &#125;</span><br><span class="line">  notify() &#123;</span><br><span class="line">    <span class="keyword">this</span>.sub.forEach(<span class="function"><span class="params">cb</span> =&gt;</span> cb());</span><br><span class="line">  &#125;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">watcher</span>(<span class="params">callback</span>) </span>&#123;</span><br><span class="line">  target = callback;</span><br><span class="line"></span><br><span class="line">  <span class="comment">// 此处触发一下监听对象的 get 方法，将 callback 方法加入 Dep 依赖的订阅队列中</span></span><br><span class="line">  <span class="comment">// 然后清除掉 target，防止重复注册</span></span><br><span class="line"></span><br><span class="line">  target = <span class="literal">null</span>;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line">watcher(<span class="function"><span class="params">()</span> =&gt;</span> &#123;</span><br><span class="line">  <span class="comment">// 此处是订阅事件</span></span><br><span class="line">&#125;)</span><br></pre></td></tr></table></figure>
<p>这样我们就讲观察者的订阅方法和 Dep 依赖给绑定上了。考虑到 watcher 的通用性，这里写了一个加强版的 demo，如下：</p>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br><span class="line">55</span><br><span class="line">56</span><br><span class="line">57</span><br><span class="line">58</span><br><span class="line">59</span><br><span class="line">60</span><br><span class="line">61</span><br><span class="line">62</span><br><span class="line">63</span><br><span class="line">64</span><br><span class="line">65</span><br><span class="line">66</span><br><span class="line">67</span><br><span class="line">68</span><br><span class="line">69</span><br><span class="line">70</span><br><span class="line">71</span><br><span class="line">72</span><br><span class="line">73</span><br><span class="line">74</span><br><span class="line">75</span><br><span class="line">76</span><br><span class="line">77</span><br><span class="line">78</span><br><span class="line">79</span><br><span class="line">80</span><br><span class="line">81</span><br><span class="line">82</span><br><span class="line">83</span><br><span class="line">84</span><br><span class="line">85</span><br><span class="line">86</span><br><span class="line">87</span><br><span class="line">88</span><br><span class="line">89</span><br><span class="line">90</span><br><span class="line">91</span><br><span class="line">92</span><br><span class="line">93</span><br><span class="line">94</span><br><span class="line">95</span><br><span class="line">96</span><br><span class="line">97</span><br><span class="line">98</span><br><span class="line">99</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">let</span> target = <span class="literal">null</span>;</span><br><span class="line"></span><br><span class="line"><span class="comment">// （私人管家）依赖收集</span></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Dep</span> </span>&#123;</span><br><span class="line">  <span class="keyword">constructor</span>() &#123;</span><br><span class="line">    <span class="keyword">this</span>.subscriberList = [];</span><br><span class="line">  &#125;</span><br><span class="line"></span><br><span class="line">  <span class="comment">// 将当前的 watcher 加入 dep 中</span></span><br><span class="line">  addSub(watcher) &#123;</span><br><span class="line">    <span class="keyword">this</span>.subscriberList.push(watcher)</span><br><span class="line">  &#125;</span><br><span class="line"></span><br><span class="line">  <span class="comment">// 添加订阅方法</span></span><br><span class="line">  depend() &#123;</span><br><span class="line">    <span class="keyword">if</span> (target) &#123;</span><br><span class="line">      target.addDep(<span class="keyword">this</span>);</span><br><span class="line">    &#125;</span><br><span class="line">  &#125;</span><br><span class="line"></span><br><span class="line">  <span class="comment">// 发布者更新消息，触发所有的订阅方法</span></span><br><span class="line">  notify() &#123;</span><br><span class="line">    <span class="keyword">this</span>.subscriberList.forEach(<span class="function"><span class="params">sub</span> =&gt;</span> &#123;</span><br><span class="line">      sub.update();</span><br><span class="line">    &#125;);</span><br><span class="line">  &#125;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="comment">// （平台）观察者</span></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Watcher</span> </span>&#123;</span><br><span class="line">  <span class="keyword">constructor</span>(data = &#123;&#125;, key = <span class="string">''</span>, cb = <span class="function"><span class="params">()</span> =&gt;</span> &#123;&#125;) &#123;</span><br><span class="line">    <span class="keyword">this</span>.cb = cb;</span><br><span class="line">    <span class="keyword">this</span>._data = data;</span><br><span class="line">    <span class="keyword">this</span>.key = key;</span><br><span class="line"></span><br><span class="line">    target = <span class="keyword">this</span>;</span><br><span class="line">    <span class="comment">// 触发 getter，存储本 watcher</span></span><br><span class="line">    <span class="keyword">this</span>.value = data[key];</span><br><span class="line">    <span class="comment">// 防止反复触发</span></span><br><span class="line">    target = <span class="literal">null</span>;</span><br><span class="line">  &#125;</span><br><span class="line"></span><br><span class="line">  addDep(dep) &#123;</span><br><span class="line">    dep.addSub(<span class="keyword">this</span>);</span><br><span class="line">  &#125;</span><br><span class="line"></span><br><span class="line">  update() &#123;</span><br><span class="line">    <span class="keyword">const</span> newVal = <span class="keyword">this</span>._data[<span class="keyword">this</span>.key];</span><br><span class="line">    <span class="keyword">this</span>.value = newVal;</span><br><span class="line">    <span class="keyword">this</span>.cb(newVal);</span><br><span class="line">  &#125;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="comment">// （发布者和管家的联系方式）监听器</span></span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">observer</span>(<span class="params">data = &#123;&#125;</span>) </span>&#123;</span><br><span class="line">  <span class="built_in">Object</span>.keys(data).forEach(<span class="function"><span class="params">key</span> =&gt;</span> &#123;</span><br><span class="line">    <span class="keyword">let</span> val = data[key];</span><br><span class="line">    <span class="keyword">const</span> dep = <span class="keyword">new</span> Dep();</span><br><span class="line"></span><br><span class="line">    <span class="built_in">Object</span>.defineProperty(data, key, &#123;</span><br><span class="line">      <span class="keyword">get</span>() &#123;</span><br><span class="line">        <span class="keyword">if</span> (target) &#123;</span><br><span class="line">          <span class="comment">// get 时添加依赖</span></span><br><span class="line">          dep.depend();</span><br><span class="line">        &#125;</span><br><span class="line">        <span class="keyword">return</span> val;</span><br><span class="line">      &#125;,</span><br><span class="line">      <span class="keyword">set</span>(newVal) &#123;</span><br><span class="line">        <span class="keyword">if</span> (newVal === val) &#123;</span><br><span class="line">          <span class="keyword">return</span>;</span><br><span class="line">        &#125;</span><br><span class="line">        <span class="comment">// set 时触发更新</span></span><br><span class="line">        val = newVal;</span><br><span class="line">        dep.notify();</span><br><span class="line">      &#125;</span><br><span class="line">    &#125;)</span><br><span class="line">  &#125;);</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="comment">// 发布者</span></span><br><span class="line"><span class="keyword">const</span> publisher = &#123;</span><br><span class="line">  bookName: <span class="string">'book'</span>,</span><br><span class="line">  bookContent: <span class="string">'hello world'</span></span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="comment">// 管家开始观测发布者</span></span><br><span class="line">observer(publisher);</span><br><span class="line"></span><br><span class="line"><span class="comment">// 订阅者在平台上订阅发布者的部分信息</span></span><br><span class="line"><span class="keyword">new</span> Watcher(publisher, <span class="string">'bookName'</span>, name =&gt; &#123;</span><br><span class="line">  <span class="built_in">console</span>.log(<span class="string">`new book name is <span class="subst">$&#123;name&#125;</span>`</span>);</span><br><span class="line">&#125;);</span><br><span class="line"><span class="keyword">new</span> Watcher(publisher, <span class="string">'bookContent'</span>, content =&gt; &#123;</span><br><span class="line">  <span class="built_in">console</span>.log(<span class="string">`new book content is <span class="subst">$&#123;content&#125;</span>`</span>);</span><br><span class="line">&#125;);</span><br><span class="line"></span><br><span class="line"><span class="comment">// 发布者发布信息</span></span><br><span class="line">publisher.bookName = <span class="string">'new book'</span>;</span><br><span class="line">publisher.bookContent = <span class="string">'new content'</span>;</span><br></pre></td></tr></table></figure>
<h3 id="Vue-观察者的实现"><a href="#Vue-观察者的实现" class="headerlink" title="Vue 观察者的实现"></a>Vue 观察者的实现</h3><p>在看源码之前，首先让我们看看 Vue 的实现原理图：</p>
<p><img data-src="/images/vue-observer.jpeg" alt="Reactive"></p>
<p>先仅考虑 data 部分的观察者模式，我们可以看到大致步骤为：</p>
<ol>
<li>为发布者 data 做数据监听处理；</li>
<li>为劫持方法添加 Dep 依赖收集，<code>get</code> 存储 target 目标，set 设置 target 目标；</li>
<li>依赖收集触发后，触发 Watch 观察者，通知订阅者改变。</li>
</ol>
<p>因为流程图中只是详细展示了响应式部分的变动，为了便于大家更清楚的理解这部分的流程，所以这里先解释一下 Vue 中完整的流程：</p>
<ol>
<li>页面开始渲染，对 data 进行依赖收集；</li>
<li>依赖收集完毕，开始监测订阅者的方法，并存储原值方便变更时的对比；</li>
<li>因为依赖收集完毕，要存储原值则触发了 get 事件，此时添加 dep 依赖，将对应生成的 watcher 存储入依赖列表内</li>
<li>页面渲染完成，用户触发页面某些点击事件（比如按钮）；</li>
<li>点击事件绑定了某个 methods 方法，该 methods 方法使得 data 部分内容更新了；</li>
<li>因为已经对 data 进行了依赖收集，触发了对应的 set 方法；</li>
<li>set 方法对比新旧值发现值更新了，此时重新收集依赖新的值（特别是对象的情况），然后通知它的私有管家 Dep 进行更新；</li>
<li>Dep 依赖开始遍历其所存储的数组，为该发布者的订阅者们发送消息，触发订阅事件。</li>
<li>各订阅者事件触发 update 更新，此时便流转到 render 渲染，之后页面就更新啦！</li>
</ol>
<p>为了方便大家理解源码，这里手动实现了一个简单的 observer 模式，基本是按着源码的思路一点点撸出来的，注释很全，方便大家理解！</p>
<ul>
<li><a href="https://github.com/kazehaiya/vue-observe" target="_blank" rel="noopener">个人版 vue-observer 源码</a></li>
</ul>
<h3 id="源码地址"><a href="#源码地址" class="headerlink" title="源码地址"></a>源码地址</h3><p>所有的 demo 都写入仓库了，仓库地址为：<a href="https://github.com/kazehaiya/vue-observe" target="_blank" rel="noopener">传送门</a></p>

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